Contact a meat processing plant (or local butcher or even grocery store) ahead of time and have them save a cow's tongue. Bring it in inside a plastic bowl or something similar and cover the bowl so that the students can't see what is inside. Then have volunteers come up and put their hands inside and try to guess what it is. After the tongue is revealed, you will have some pretty funny reactions. Be prepared to have kids wash their hands thoroughly after they do this. It also adds to it to put some kind of gel on the tongue for more grossness.